ok this has happened to me just recently and I have been sewing for years but never had this problem. I checked everything I could think of the bobbin, top thread all looked OK so I even took the bobbin out un-threaded it and re done that. I took the whole machine apart and did the whole maintenance thing, you know the oiling, cleaning and such.
Started sewing again, after a few stitches it did it again. Hammer was ready to give it it's last rights then I looked at the top thread and found it had wrapped itself around the top guide in such a way that it formed a loop and would not let the thread run freely. I have never had any thread do this before but I had bought a large spool of serger thread so I wouldn't have to change it so often and it has been the worst thread I have ever had. I have even tried putting it on a thread stand by itself but it still tangles itself round different parts of the machine while I sew and every time I try to wind a bobbin it flings itself so wildly it is unbelievable. So I would check to be sure this is not happening to you as well.